ISIS claimed responsibility for the attacks Saturday in a statement that warned, “Indeed, this is just the beginning.”

The scene inside the Bataclan concert hall minutes after the Paris terror attacks.Mirrorpix.com

“This is what we had feared,” the Wall Street Journal quoted a senior French official saying of the specter of terrorists hiding among the 800,000 Syrian refugees who have flooded into Europe so far this year.

At least one of the terrorists was found with a Syrian passport, presumably having made his way to France via Greece as a refugee last month.

A second attacker also may have presented himself as a refugee on the Greek island of Leros two months earlier, officials said.

Authorities wouldn’t rule out the possibility that other extremists tied to the attack remain on the loose.

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If the attack did involve militants who traveled to Europe amid hundreds of thousands of refugees from the Middle East, the implications could be profound as the debate over Europe’s immigration policy reaches a fever pitch.

The attack brought an immediate tightening of borders as President François Hollande, calling the attacks “an act of war,” declared a state of emergency and announced renewed border checks.

Germany also stepped up border checks, but Chancellor Angela Merkel seemed to hold onto her stance of placing no limits on the number of people her country was willing to give refuge.

“[ISIS is] now going after Western institutions — people who are living life…”

Poland’s prospective minister for European affairs, Konrad Szymanski, said that in light of the attacks, his country would not comply with an EU plan to accept refugees unless it received “guarantees of security.”

Brian Michael Jenkins, a terrorism expert, made the stakes clear for all of Europe’s leaders: “The big question on everyone’s mind is: Were these attackers — if they turn out to be connected to one of the groups in Syria — were they homegrown terrorists or were they returning fighters?” he asked.

Asylum-seekers fleeing Syria and other war-torn countries condemned the attacks, fearing it would become even more difficult now to start new lives in Western Europe.

Members from U2 pay homage to attacks’ victims near the Bataclan concert hall.AFP/Getty Images

Zebar Akram, 29, from Iraq, said, “This is the same act of terrorism like they act in Syria or Iraq.”

Abdul Selam, 31, of Syria, said he feared that refugees now “will be considered as probable attackers.”

French prosecutor François Molins said the three jihadi teams that carried out the coordinated attacks included at least one cell with ties to Belgium, where three people were arrested Saturday — including a Frenchman who had rented a Volkswagen Polo found near the Bataclan, where 89 were slain.

The raids in the Brussels suburb of Molenbeek came after investigators traced two cars, including the Polo, to Belgium. The second car, with Belgian plates, was seen near a cafe that was targeted.

Authorities believe the atrocity was planned by a network stretching from Syria and the Mideast to Germany, France and Belgium, the Sunday Times of London reported.

A US official briefed on the matter said the attack may indicate a shift in ISIS’s strategy and is a “cause for alarm to all Westerners.”

Belgian police cordon off a street in Molenbeek on November 14, 2015.AFP/Getty Images

“They are now going after Western institutions — people who are living life, going to concerts, sports events, going to restaurants. This is very symbolic of life in the West and something that ISIS detests and is repulsed by under their own set of values,” the official told The Post.

Prosecutor Molins said 99 of the wounded were in critical condition after the “act of barbarism,” in which the terrorists shouted, “Allahu akbar” (“God is great”) as they fired AK-47s, threw grenades and finally blew themselves up with suicide vests.

The terrorists at the concert hall mentioned Syria and Iraq, Molins said. Investigators have recordings of two of them at the hall as they spoke in French, El Figaro reported.

“They didn’t stop firing,” Pierre Janaszak, a radio presenter who was at the concert, told Agence France-Presse. “There was blood everywhere, corpses everywhere. Everyone was trying to flee.”

The terrorists were heard raging at Hollande and his decision in September to begin airstrikes on ISIS in Syria.

People place flowers and light candles in tribute for the victims of the Paris attacks.EPA

“I clearly heard them say, ‘It’s the fault of Hollande, it’s the fault of your president, he should not have intervened in Syria,’ ” Janaszak added.

ISIS made light of the West in its statement.

“A group of believers from the soldiers of the Caliphate (may Allah strengthen and support it) set out targeting the capital of prostitution and vice . . . Paris,” the group said.

The statement mocked France’s involvement in airstrikes on suspected ISIS bases in Syria and Iraq, noting France’s air power was “of no use to them in the streets and rotten alleys of Paris.”

The attacks were “prepared, organized and planned overseas, with help from inside [France], which the investigation will establish,” according to Hollande.

Eurasia Group, a leading global political-risk research firm, reported that the attacks “confirm a structural shift in the modus operandi of the Islamic State, and represent a prelude to additional attacks in the West.”